Test that MockDirectory.CreateDirectory supports windows extended-length paths

Windows paths can have a `\\?\` prefix, the '?' character is not always invalid.
Ref: https://learn.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation?tabs=registry

Refers to #1304

Author: mostynb

tests/TestableIO.System.IO.Abstractions.TestingHelpers.Tests/MockDirectoryTests.cs

@@ -747,6 +747,20 @@ public async Task MockDirectory_CreateDirectory_ShouldThrowIfIllegalCharacterInP
         await That(createDelegate).Throws<ArgumentException>();
     }
 
+    [Test]
+    [WindowsOnly(WindowsSpecifics.UNCPaths)]
+    public async Task MockDirectory_CreateDirectory_ShouldSupportExtendedLengthPaths()
+    {
+        // Arrange
+        var fileSystem = new MockFileSystem();
+
+        // Act
+        fileSystem.Directory.CreateDirectory(XFS.Path(@"\\?\c:\bar"));
+
+        // Assert
+        await That(fileSystem.Directory.Exists(XFS.Path(@"\\?\c:\bar"))).IsTrue();
+    }
+
     // Issue #210
     [Test]
     public async Task MockDirectory_CreateDirectory_ShouldIgnoreExistingDirectoryRegardlessOfTrailingSlash()
@@ -2219,4 +2233,4 @@ public static void MockDirectory_Move_ShouldNotThrowException_InWindows_When_Sou
         // Act & Assert
         Assert.DoesNotThrow(() => mockFs.Directory.Move(src, dest));
     }
-}
+}